The 1-2 shift solenoid contains a plunger mechanism that the ECU/TCM uses to route pressurized transmission fluid inside the valve body in order to actuate the 1-2 shift. If an unusually high voltage signal is detected (usually caused by a short or open circuit), then a P1843 diagnostic trouble code will be stored.
How Serious is the Code?
The P1843 DTC is a serious code, as it relates the very functionality of the transmission. If the 1-2 shift solenoid problem is not dealt with promptly, the vehicle will become undrivable.
Symptoms of a P1843 include:
- Unusually harsh shifts
- Delayed shifts
- The vehicle refuses to move forward
- Limp mode
- Engine stalling
- Transmission overheating
- Low transmission fluid level
- Dirty transmission fluid/filter
- Blocked hydraulic passages
- Damaged or corroded wiring/connectors
- Faulty 1-2 shift solenoid
- Faulty transmission pump
How to Diagnose the Code?
The first thing that a mechanic should do when diagnosing a P1843 OBDII diagnostic trouble code, is check the condition and level of the transmission fluid. Dirty transmission fluid can lead to blocked hydraulic passages, which can affect the operation of the 1-2 solenoid. If the ATF is clean and bright, then the wiring in connector should be examined for signs of a short or corrosion. Should no problems be found, then the 1-2 shift solenoid should be suspected.
Common Mistakes When Diagnosing
It’s not uncommon for most mechanics to suspect that a P1842 DTC automatically means that the 1-2 solenoid has failed. When many times, could just be dirty transmission fluid or a wiring issue.
What Repairs Will Fix P1842?
- Changing the transmission fluid/filter
- Repair or replace damaged wiring/connectors
- Replace the 1-2 shift solenoid
- Repair or replace the transmission pump
- Repair or replace the valve body
P1842 – 1-2 Shift Solenoid Low Voltage